docs: document OpenCode plugin cache location and upgrade flow

OpenCode loads plugins from ~/.cache/opencode/packages/<name>@<spec>/,
not from ~/.config/opencode/node_modules/ as we previously assumed.
Updating the wrong directory is why our 0.1.1/0.1.2/0.1.3 publishes
had no effect on the running instance — OpenCode kept loading 0.0.45
from its own cache.

docs/opencode-plugin-cache.md:
- Document the real plugin loading path
- Explain the red-herring ~/.config/opencode/node_modules/ directory
- Three-step upgrade procedure (cache flush + proxy kill + OpenCode restart)
- Debugging symptom table (how to spot a stale-cache situation)
- Version SHA reference table for 0.0.45, 0.1.1, 0.1.2, 0.1.3

notepad:
- Agent continuity note with evidence snapshot and rationale